A communications method as a successor of W-CDMA and HSDPA, namely, Long Term Evolution (LTE) has been considered by a W-CDMA standardization organization 3GPP. As a radio access method, Orthogonal Frequency Division Multiplexing (OFDM) is under consideration for downlink, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) is under consideration for uplink (see Non-patent Publication 1, for example).
In OFDM, a frequency band is divided into plural narrow frequency bands (sub-carriers), and data are placed on the respective divided frequency bands to carry out transmission. The sub-carriers are densely arranged in a frequency direction, allowing the sub-carriers to be partly overlapped without causing interference, thereby realizing high speed transmission and improving frequency usage efficiency.
In SC-FDMA, a frequency band is divided into plural narrow bands, and different narrow bands are used by different terminal devices, so that interference between the user terminals can be reduced. According to SC-FDMA, which is characterized in that variations in the transmission electric power are reduced, a large coverage area and low energy consumption can be realized.
A mobile communications system utilizes indefinite radio resources (frequency, power) to carry out communications, and there is an upper limit of communications capacity. Therefore, the number of mobile stations in a cell has to be limited depending on the communications capacity. For example, when a mobile station tries to start communications anew in the cell, if a large number of mobile stations are carrying out communications in the cell and the communications capacity reaches near the upper limit, the new mobile station has to be controlled so that the new communications are not allowed. More specifically, call admission control can be thought that does not allow the new mobile station to start communications anew when the number of the mobile stations carrying out communications in the cell is counted and the number exceeds a predetermined threshold value, and allows the new mobile station to start communications anew when the number is less than or equal to the predetermined threshold value. From a viewpoint of such call admission control, the number of the mobile stations carrying out communications has to be the number of the mobile stations that are consuming the radio resources.
In addition, there are generally plural carriers in a communications system. For example, when one communications system has a frequency bandwidth of 20 MHz and provides a communication service using W-CDMA in the 20 MHz frequency bandwidth, there exist four W-CDMA carriers because a frequency bandwidth of one carrier in W-CDMA is 5 MHz. In this case, it is preferable from a viewpoint of efficient usage of the frequency resources that the number of mobile stations be equal in each of the four W-CDMA carriers.
Further explanation is made about the number of the mobile stations.
For example, W-CDMA utilizes a dedicated channel individually established between the mobile station and the base station apparatus, and a power resource and a code resource, which are consumed radio resources, are proportional to the number of the dedicated channels. Therefore, the number of the mobile stations carrying out communications in the cell is thought to be the same as the number of mobile stations for which the dedicated channel is established. In addition, the number of mobile stations for which a connection between the mobile station and the base station apparatus is established and the number of mobile stations to which a dedicated channel is established are generally the same.
On the other hand, LTE utilizes shared channels in uplink and in downlink, and the power resource and the code resource, which are consumed radio resources, are shared by plural mobile stations. This means that no radio resources are ensured for the individual mobile station. As a result, it becomes difficult that the number of the mobile stations having connection established with the base station apparatus corresponds one-to-one with the radio resources to be consumed. In addition, because LTE mainly intends packet data transmission, there may be a mobile station that does not consume any radio resources even when the connection is established between the base station apparatus and the mobile station, depending on an occurrence pattern of the packet data. For example, it can be imagined that a mobile station may download web contents in an LTE system only for three minutes out of 20 minutes during which the connection between the mobile station and the base station apparatus is established. Even in this case, it becomes difficult that the mobile stations having connection established with the base station apparatus correspond one-to-one with the radio resources to be consumed. In LTE, a state where the connection between the mobile station and the base station apparatus is established is called an LTE active state or an RRC connected state.
Moreover, it is under consideration that the LTE active state be divided into a state where downlink data are continuously received and a state where downlink data are discontinuously received (see Non-patent Document 2, for example). A state that satisfies the LTE active state and in which downlink data are discontinuously received is called a Discontinuous Reception (DRX) state. Because a mobile station in the DRX state consumes fewer radio resources, it becomes difficult for the mobile stations having connections established with the base station apparatus correspond one-to-one with the radio resources to be consumed.    Non-patent Document 1: 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006.    Non-patent Document 2: 3GPP TR 36.300 (V0.3.1), “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2”, September 2006.